The sleep disorders are organized into four major sections according to presumed etiology. Primary Sleep Disorders are those in which none of the etiologies listed below (i.e., another mental disorder, a general medical condition, or a substance) is responsible. Primary Sleep Disorders are presumed to arise from endogenous abnormalities in sleep-wake generating or timing mechanisms, often complicated by conditioning factors. Primary Sleep Disorders in turn are subdivided into Dyssomnias (characterized by abnormalities in the amount, quality, or timing of sleep) and Parasomnias (characterized by abnormal behavioral or physiological events occurring in association with sleep, specific sleep stages, or sleep-wake transitions).